The present invention relates to a process for the production of powder catalysts comprising titanated magnesium chloride and to a precursor for such a catalyst.
U.S. Pat. No. 4,650,778 discloses the production of metal halide particles useful as olefin polymerization catalyst precursors by vaporising the metal halide and then condensing it in the presence of a diluent.
It is also known for example from EP-A-0654444 and EP-A-0654485 to produce powder catalysts based on magnesium chloride, titanium chloride and at least an electron donor by using a plasma torch which vaporizes the compounds introduced into the plasma torch and the vapour is condensed to form a fine magnesium chloride powder covered with titanium chloride. The powder is employed as a catalyst in the polymerisation of alpha-olefins.
While those latter two prior specifications, both of which are in the name of the present applicant, disclose the production of catalysts having good activity in the polymerisation of alpha-olefins, nevertheless there is still the demand for such catalysts with yet higher activity.
U.S. Pat. No. 4,636,486 discloses a chemical preparation process for a magnesium halide supported Ziegler-Natta catalysts, the most active anhydrous magnesium chloride is obtained when its x-ray diffraction spectrum is characterised by a broadened halo appearing at a lattice distance (d) within the range of from 2.56 to 2.95 Angstroms. This may be compared to a very crystalline magnesium chloride which exhibits an intense diffraction line at a lattice distance (d) of 2.56 Angstroms. Thus this prior patent document discloses that a polymerisation catalyst based on anhydrous magnesium dichloride has improved activity.
The chemical process disclosed in this document is however a multi-step procedure which is more expensive than the plasma torch processes disclosed in the two European specifications mentioned above.
The present invention provides a process for the production of a magnesium chloride powder for use in a catalyst, the process comprising vaporising magnesium chloride in a plasma torch and quenching the vapor with a liquid containing an electron donor to form a magnesium chloride-based powder wherein at least 80% by weight of the magnesium chloride is present as the hexagonal phase thereof.
Preferably, the electron donor is dissolved in an organic diluent in an amount of at least 2 vol % based on the amount of diluent.
Preferably, the electron donor is present in an amount of from 2.5 to 10 vol % based on the amount of diluent.
Preferably, the electron donor is selected from tetrahydrofuran (THF), 1,3 dioxolane, dioxane, diethyl ether, methyl tert-butyl ether, diisobutyl phthalate, di-n-butyl phthalate, ethyl n-butyl phthalate, diethyl phthalate, ethyl i-butyl phthalate and mixtures thereof.
Preferably, the vapour is quenched at a temperature of below xe2x88x9210xc2x0 C. to condense the magnesium chloride powder. The quenching step may be followed by a titanation step in which the magnesium chloride powder is titanated to form a catalyst for polymerising alpha-olefins.
Preferably, the titanated catalyst has a median particle size d50 of from 5 to 50 microns and more preferably from 8 to 35 microns.
Preferably, the titanated catalyst has an activity in the polymerisation of polyethylene of at least 20000 grams of polyethylene per gram of catalyst per hour.
The present invention provides a precursor for a magnesium chloride-based catalyst for polymerising alpha-olefins, the precursor comprising a magnesium chloride/tetrahydrofuran complex wherein at least 80% by weight of the magnesium chloride is present as the hexagonal phase thereof.
In contrast to the teaching of U.S. Pat. No. 4,636,486, the present inventor has discovered that in the manufacture of a magnesium chloride-based polymerisation catalyst the use of magnesium chloride powder having a significant proportion, typically at least 80% by weight, of the hexagonal crystalline form of the magnesium chloride, and thus having a very crystalline structure, can result in this very crystalline structure being transformed into a highly active catalyst by subsequent treatment by titanation, for example by titanium tetrachloride. If the crystalline structure of the magnesium chloride powder is less than 100% hexagonal, the remainder of the magnesium chloride is present in the rhombohedral crystalline phase. The present inventor has discovered that catalysts prepared using rhombohedric magnesium chloride have a relatively low activity in alpha-olefinic, e.g. ethylene, polymerisation as compared to catalysts prepared using hexagonal magnesium chloride. The hexagonal form of magnesium chloride tends to be less stable than the rhombohedric form. In accordance with the present invention, the magnesium chloride powder is formed by a plasma torch technique employing particular quenching conditions which increase the yield of the hexagonal phase of the magnesium chloride powder by forming a stable complex of the hexagonal magnesium-chloride with an electron donor, such as, for example, tetrahydrofuran (THF). This acts as a precursor for the subsequently titanated catalyst. In addition, the quenching conditions can be controlled to yield high catalyst activity in the resultant titanated catalyst, with a low median particular size of the catalyst powder.